Method of making a screen stencil



United States Patent I 1,064,166 6/1913 Pindikowskyt David C. Hellman Kokomo, Indiana April 11, 1968 Nov. 10, 1970 General Motors Corporation Detroit, Michigan a corporation of Delaware inventor Appl. No. Filed Patented Assignee METHOD OF MAKING A SCREEN STENCIL 5 Claims, 5 Drawing Figs.

11.8. C1 101/ 128.3 Int. Cl B301) 9/06 Field ofSearch 101/1282, 128.3

References Cited UNlTED STATES PATENTS 1,371,157 3/1921 Gestetner 101/1283 2,969,732 1/1961 Kendall 101/l28.3

FORElGN PATENTS 420,125 11/1934 Great Britain 101/1283 Primary Examiner--Robert E. Bagwill Assistant Examiner-Alan E. Kopecki Attorney-Warren D. Hill, Jean L. Carpenter and Paul Fitzpatrick ABSTRACT: A stencil is formed in a sheet of metal and secured to a silk screen coated with a photoresist emulsion. The emulsion serves as the bonding agent. The stencil is applied to the screen while the emulsion is wet and pressure is applied until the emulsion dries. The emulsion is then removed in the areas corresponding to the stencil openings.

Patented Nov. 10,1970 I 3,538,847

INVENTOR.

A T TORNE Y This invention relates to a method of making screen stencils and particularly to a method of applying a metallic'stencil to a screen. Ordinarily, silk screens are made by coating a screen material with a photosensitized emulsion, selectively exposing portions of the screen to light and then developing the screen to remove the emulsion from certain areas of the screen to produce a desired pattern. The screens produced are generally satisfactory for a limited number of printings but eventually, the emulsion is degraded so that the integrity of the pattern is lost. When such screens are used for making printed circuits, such as thick film circuits where conductors and resistors and the like are printed by a silk screen, it is very important that not only the pattern maintain its shape, but also the screen must maintain a certain thickness so that the material deposited will have a specified thickness. In practice, wearing of the emulsion takes place when abrasive printing materials are used and the thickness of the screen rapidly decreases to the point where the screen is no longer useful.

To improve the useful life of a screen, it is desirable to reinforce the screen with a stencil formed from a solid sheet of metal. The pattern of the stencil must be aligned with the pattern on the screen and the stencil must adhere to the screen well enough to last for several thousand printings. In addition, the screen stencil must be economical to fabricate.

The general object of this invention is to provide a method of making a screen stencil meeting the above requirements.

Another object of the invention is to provide a method of adhering a metal stencil to a screen.

A further object of the invention is to provide a method of adhering a stencil to a screen coated with photoresist emul- SlOIl.

The invention is carried out by using a photoresist emulsion as the material for bonding a metal stencil to a screen.

The invention is further carried out by'forming a stencil from a thin sheet of metal, coating ascreen with a coating of photoresist emulsion, pressing the stencil onto the coated screen while the emulsion is wet and then drying the emulsion and removing the emulsion from the openings of the stencil.

The above and other advantages of the invention will be made more apparent from the following description and the accompanying drawings wherein like reference numerals refer to like parts and wherein:

FIG. 1 is an exploded perspective view of an emulsion coated screen and a metal stencil illustrating one step of the process;

FIG. 2 is a sectioned perspective view showing the screen and stencil and means for pressing them together in another step of the process;

FIG. 3 is a sectioned perspective view of the screen and stencil and a positive transparency illustrating the exposure step of the process;

FIG. 4 is a sectioned perspective view of the screen and stencil illustrating the development step of the process; and

FIG. 5 is a perspective view of a screen stencil made by the subject process. 7

The method of this invention is ada pted to make screen stencils for any purpose, but is particularly intended to make screen stencils for printing electrical circuits.

Although printing screens of the character described herein are generically termed silk screens, the actual screen material may be, for example, silk, nylon, stainless steel or other material depending on the intended use. For making printed circuits stainless steel screens are generally preferred.

In the first step of the method, a stencil 10 having an aperture 12 or apertures corresponding to the pattern to be printed is formed in a sheet of solid metal. The material of the sheet is preferred to be stainless steel, but brass, copper or other materials are suitable. The thickness of the metal should be in the range of .001 to .004 inch. The stencil pattern may be formed by any desired means, however a method of photoetching is preferred.

Means for applyingpressure .to the screen and stencil as- As shown in FIG. 1, a screen 14 larger than the stencil 10 is thoroughly coated with a photosensitized emulsion 16 which is generally known as a photoresist emulsion and then the stencil '10 is placed on the emulsion while the emulsion is wet. Then, uniform pressure is applied to the stencil and screen assembly,

and theemulsion is allowed to dry while under'pressure.

sembly are shown in FIG. 2. A sheet of Mylar 18 is placed on each side of the'assembly and a flat metal weight 20 larger than the stencil is placed over the stencil on the upper sheet of Mylar 18. The whole stack of materials rests on a glass sheet 22. The Mylar sheets 18 protect the emulsion 16 and prevents it from sticking to the weight 20 and glass sheet 22. The emulsion is allowed to dry at room temperature, or, if a shorter drying time is desired, at a slightly elevated temperature. Then when the'emulsion is dry, the screen and stencil assembly is removed from between the Mylar sheets and a positive transparency 24 of the stencil pattern is placed against the coated screen on the side opposite the stencil as shown in 1-10.}. The

transparency 24 is opaque in the portion 24a corresponding to the stencil aperture 12 and is transparent in the portion 24b in the area outside the aperture 12. The emulsion 16 is selectively exposed to light through the transparency so that the emulsion outside the area defined by the stencil aperture 12 is hardened. After removal of the transparency 24, the emulsion I6 is developed as shown in FIG. 4 by washing away the unexposed emulsion with a solvent. The resulting screen stencil is illustrated in FIG. 5 wherein it is apparent that except for the area defined by the stencil aperture 12, the entire screen 14 is coated with anemulsion 16 and the metal stencil 10 secured to the screen reinforces the emulsion pattern to maintain the integrity of the configuration and especially serves to maintain the thickness of the .ink or paste deposited through the screen.

The stencil 10 is bonded to the screen 14 by the emulsion 16 but if desired, the assembly may be reinforced by applying a bead 26 of-emulsion or epoxy cement around the outer edge of the stencil l0.

While many types of photoresist-emulsion are commercially available, andcan be used in this method, it has been found that those emulsions which assume a brittle enamellike condition are not always satisfactory bonding agents because in use, the screen stencil flexes slightly creating considerable stress in the bond and the stencil may tend to become loose. It is much preferred to use a plastic emulsion which, when hardened, is resilient or flexible thereby providing a pliable bond for the flexible assembly. One such plastic emulsion comprises a commercially available mixture of poly-vinyl acetate and polyvinyl alcohol. This mixture is photosensitized with a photosensitizing agent by adding 6 grams of a 6 percent solution of am monium dichromate per l00 grams of the mixture.

The method of this invention has been used in practice as follows. A desired stencil pattern is formed in a 2 inch by 3 lowed to dry. A central area of the screen slightly larger than the stencil 16 is masked and the screen is exposed to light and developed by washing in water to remove the unexposed emulsion from the masked portion of the coated screen 14 and to leave the exposed emulsion on the remainder of the screen.

. More emulsion is applied to the central area of the screen and is thoroughly worked into the screen, the excess being "removed by a squeegee. The stencil 10 is applied to the wet emulsion l6 and is placed under pressure as illustrated in FIG 2 using a 2 pound metal weight 20. The assembly is heated in an oven at F for 1 minute, then the weight 20 and the Mylar sheets 18 are removed and the stencil screen laminate is left in the oven for 5 minutes. Then two more coats of emulsion are applied to the screen on the side of the screen opposite the stencil and each in turn is dried in the same manner. Prior to exposing the newly applied emulsion, an overexposed positive transparency of the pattern is aligned with the stencil aperture 12 on the opposite side of the screen 34. Because the transparency is overexposed, the opaque portion of the pattern will be .001 or .002 inch larger than the aperture 12. This assures that in the finished screen stencil the emulsion will not protrude into the area of the aperture 112. On the other side of the screen, the stencil aperture is covered with a mask and then both sides of the screen are exposed to light to harden the unprotected photoresist emulsion. The transparency and the mask are removed and the screen stencil laminate is then washed with water to remove the unexposed emulsion.

Screen stencils formed by this method have been found in practice to be far superior to conventional screens which rely on emulsion only to define the pattern in that the integrity of the pattern is maintained for a longer period of use and the thickness of the printing materials applied through the screen is held constant.

The embodiment of the invention described herein is for purposes of illustration and the scope of the invention is intended to be limited only by the following claims:

l claim:

1. The method of making a screen stencil comprising the steps of:

forming a stencil from a sheet of stainless steel having a thickness in the range of about 0.001 to 0.004 inch by coating the sheet with a photoresist emulsion, selectively exposing the photoresist to light through a positive transparency of a desired pattern, washing away the unexposed photoresist emulsion, and etching the sheet in fer ric chloride;

coating a stainless steel screen with a plastic photoresist emulsion comprising a mixture of poly-vinyl acetate, poly-vinyl alcohol and a photosensitizing agent; then laminating the stencil to the screen by placing the stencil on one side of the screen while the plastic photoresist emulsion is wet and applying uniform pressure to the stencil and screen and heating the stencil and screen at about l25F until the plastic photoresist emulsion dries;

then selectively hardening the plastic photoresist emulsion on the screen by placing a positive transparency of the said pattern adjacent the other side of the screen in alignment with the stencil pattern and exposing the plastic photoresist emulsion to light through the transparency; and

-removing the unexposed photoresist from the screen by washing with water.

2. The method of making a screen stencil comprising the steps of:

forming a stencil from a thin sheet of stainless steel by coating the sheet with a photoresist emulsion, selectively exposing the photoresist to light through a positive transparency of a desired pattern, washing away the unexposed photoresist emulsion, and etching the sheet in ferric chloride;

coating a stainless steel screen with a plastic photoresist emulsion;

then laminating the stencil to the screen by placing the stencil on one side of the screen while the plastic photoresist emulsion is wet and applying uniform pressure to the stencil and screen until the plastic photoresist emulsion dries;

then selectively hardening the plastic photoresist emulsion on the screen by placing a positive transparency of the said pattern adjacent the other side of the screen in alignment with the stencil pattern and exposing the plastic photoresist emulsion to light through the transparency; and

washing the unexposed photoresist from the screen.

3. The method of making a screen stencil comprising the ste sof:

orming a stencil from a thin sheet of metal according to a desired pattern;

coating a metallic screen with a plastic photoresist emulsion;

then laminating the stencil to the screen by placing the stencil on one side of the screen while the plastic photoresist emulsion is wet and applying uniform pressure to the stencil and screen until the plastic photoresist emulsion dries;

then selectively hardening the plastic photoresist emulsion on the screen by placing a positive transparency of the said pattern adjacent the other side of the screen in alignment with the stencil pattern and exposing the plastic photoresist emulsion to light through the transparency; and

washing the unexposed photoresist from the screen.

4. The method of making a screen stencil comprising the steps of:

forming a stencil from a thin metal sheet according to a desired pattern;

coating a screen with a photoresist emulsion;

adhering the stencil to the screen by pressing the stencil onto the emulsion coated screen and drying the emulsion;

hardening the emulsion outside the area defined by the aperture by exposing the emulsion to light through a positive transparency of the pattern aligned with the stencil pattern; and

removing the emulsion from the screen in the area defined by the stencil pattern by washing away the unexposed emulsion.

5. The method of making a screen stencil comprising the steps of:

forming a stencil by making an aperture in a sheet of matericoating a screen with a photoresist emulsion;

adhering the stencil to the coated screen by pressing the stencil onto the coated screen and drying the emulsion;

hardening the emulsion outside the area defined by the aperture by selectively exposing the emulsion to light; and

removing the emulsion from the screen in the area defined by the aperture. 

